The Kremlin has consistently denied assertions by the West that it is arming, supporting and training separatists fighting to establish an independent state in eastern Ukraine.

U.S. officials said in July they had identified a separatist training facility near Rostov, and a surface-to-air rocket that turned up in east Ukraine in June originated from a base near Moscow, according to its logbook.

On Russia's border with Ukraine, some 170 kilometers north of Rostov, there is further evidence pointing to Russia's role in the insurrection.

A gap a few meters wide has been cut in the razor-wire fence opening a route for vehicles and pedestrians to the rebel-controlled area of eastern Ukraine.

On the Russian side of the divide, a track leads towards the Russian town of Donetsk, 1 km away and bearing the same name as the rebel stronghold in Ukraine. The soil has the imprints of caterpillar treads, the kind used on tanks or some types of armored personnel carriers.

When Reuters visited this section of border in late July and early August, two Russian border guards emerged from bushes and waved the reporter away. "It's better not to drive here", one said.

The guards denied that armored personnel carriers or tanks had crossed into Ukraine along the rough track.

A kilometer away, on the outskirts of the Russian town of Donetsk, caterpillar treads have left white marks on the roadside.

UNMARKED CAMP

The trail continues several kilometers along the Seversky Donets river to an area where a military camp and training ground is hidden from the road by trees, between the village of Malaya Kamenka and the town of Kamensk-Shakhtinsky.

No identifying flags or signs marked the camp.

Its entrance was blocked by "hedgehog" anti-tank obstacles and guarded by three uniformed men without insignia. An armored personnel carrier was parked in a nearby trench, covered with a camouflage net, another was hidden in bushes nearby.

"It is a camp. Exercises are going on," one of the men said.

Russia denies the existence of training camps for Ukraine separatists. A spokesman for the border guard, Vasily Malayev, said the camp in question was a border guard unit.

In late July armored personnel carriers and a military truck were seen moving through the camp. Two cars drove away carrying men in camouflage T-shirts.

One of the cars had the regional code 95 on its plates, generally associated with Chechnya, the other bore the code 15, associated with North Ossetia. A car stripped of its plates - a tactic used by rebels in Ukraine - drove into the camp. Malayev, the border guard spokesman, said no armored cars had crossed into Ukraine.

"This is an agricultural area and traces of caterpillars don't necessarily mean tank maneuvers. A tractor may have gone there."

He said the gap in the fence was designed for local residents. "People have their dachas there. They go to their dachas. Residents have passes to go in that direction. They don't drive there, they only walk."

Ukraine said on Monday Russia had amassed 45,000 troops on its border backed by heavy equipment including tanks, missile systems, warplanes and attack helicopters.

Residents of the Russian town of Donetsk confirmed seeing armored cars in town. A Reuters reporter saw dozens of Russian military trucks on highways around Rostov, including one carrying a tank.